Storage tube with composite target consisting of display phosphor, porous dielectric and metallic membrane collector



Feb. 6', 1968 Filed July 20, 1965 AND METALLIC MEMBRANE COLLECTOR 2Sheets-Sheet 1 Richard L. Sjberg,

Nobuo J. Kode,

Leon S. Yuggy, INVENTORS.

ATTORNEY.

Feb- 6, 1968 Fe. SJBERG ETAL 3,368,093

' STORAGE TUBE WITH COMPOSITE TARGET CONSISTING OF DISPLAY PHOSPHOR,POROUS DIELECTRIC AND METALLIC MEMBRAN COLLECTOR Filed July 20,' 1965 2Sheets-Sheet 2 l oo- \\wnafe smble pom V VCOH vao- T eo 0 u' O E :s v40- 2 D c E E m :E

2O cn Black subie poin OI l l u IOO |50 200 Storage surface poieniol(volts) Fig. 5.

Collectov potenicxi lume conduchon curve ombinec w\ VP Second svfxbi@curve potenila Coilecior potential Gain M Secqndiry ermsslon UnitedStates Patent lice 3,363,093 Patented Feb. 6, 1968 STORAGE TUBE WITHCOMPOSHE TARGET CONSISTING F DISPLAY PHOSPHUR, P0-

ROUS DELECTRIC AND METALLKC MEM- BRANE COLLECTGR Richard L. Sjberg,Oceanside, Nobile J. Kuda, Vista, and Leon S. Yaggy, North Carlsbad,Calif., assignors to Hughes Aircraft Company, Culver City, Calif., acorporation of Delaware Filed .luiy 2t), 1965, Ser. No. 473,324 1 Claim.(Cl. 313-68) ABSTRACT F THE DESCLSURE A direct-viewing storage tubehaving a porous dielectric layer for storage purposes disposed directlyon the phosphor layer constituting the viewing screen.

This invention relates to direct-viewing cathode ray storage tubes,particularly of the bistable type. More particularly, the inventionrelates to a bistable direct-viewing storage tube not requiring amesh-type storage target.

A direct-viewing storage tube is a cathode ray tube in which the outputis visual and in the form of a bright and persistent display of what isgenerally nonrecurrent pictorial information. Such tubes are utilizedfor the persistent display of such types of information as radar orsonar data, processed alpha-numeric information, narrow-band televisionpictures and transient waveforms (as in oscilloscope applications).There are two principal kinds of direct-viewing storage tubes: thebistable type and the halftone type. The bistable or two-tone tubepresents displays in two shades of grey (black and white) and utilizesregeneration to provide an indenitely long retention time. The halftonetube provides a continuous range of grey shades but usually has alimited retention time. The present invention relates to direct-viewingstorage tubes of the bistable or two-tone type and prior art tubes ofthis kind and their operation are described in U.S. Patent No. 2,788,466to S. Hansen.

In general, such direct-viewing storage tubes comprise four essentialelements: a storage target or mesh, an adjacent phosphor viewing screen,a iiood electron gun and a writing electron gun. Almost all previouslyknown storage tubes of this type have employed a storage target which isa fine metal screen with several hundred holes per linear inch andhaving a thin layer of an electrically insulating dielectric materialdeposited on the side of the screen facing the iiood electron gun. Theelectrons from the ood gun cover or blanket the surface of the storagetarget in a uniform broad beam of slow electrons. By scanning thestorage target with an electron beam of elemental cross-sectional areaproduced by the writing gun, a charge pattern corresponding to thepicture to be displayed is formed on the surface of the dielectric layerby secondary electron emission, for example. Each elemental area of thecharge pattern controls the transmission of electrons from the ilood gunthrough the storage target much like the control grid of any triodevacuum tube controls the plate current. In areas of the dielectric layerwhere the charge pattern is more positive, relative to the flood guncathode, dood electrons penetrate the storage mesh and impinge upon thephosphor viewing screen. In areas where the pattern is negative relativeto the ilood gun cathode, the flood electrons are repelled by thestorage target and cannot penetrate therethrough to reach the viewingscreen. Flood electrons which do pass through the storage target areaccelerated so that they strike the phosphor viewing screen at a highenergy and produce a light pattern corresponding to the charge patternon the dielectric storage surface. As long as the charge pattern remainsundisturbed, the display persists.

It Will be noted, therefore, that such direct-viewing storage displaytubes have heretofore required the utilization of a storage targetcomprising a ne mesh screen support electrode member having a dielectriccoating thereon separately disposed intermediate the necessary electronguns and the viewing screen.

It is therefore an object of the instant invention to provide animproved direct-viewing storage tube.

Another object of the invention is to provide an improved direct-viewingstorage tube which does not require a separate mesh storage target.

Still another object of the invention is to provide an improveddirect-viewing storage tube having a simplified storage targetstructure.

These and other objects and advantages of the invention are realized byproviding a storage display tube with a combination Storage and viewingtarget structure in which a layer of porous (low density) dielectricmaterial is formed directly on the light-producing phosphor layer. Amesh electrode for collecting secondary emitted electrons is alsoprovided in the tube either adjacent to or in contact with the porousdielectric layer. With a dielectric layer of sufficient porosity iloodelectrons arriving at areas thereof which are positive with respect tothe flood gun will penetrate the dielectric layer and strike the viewingscreen or phosphor layer, thus causing iluorescence.

The invention will be described in greater detail by reference to thedrawings in which:

FIGURE l is a partly schematic, partly cross-sectional elevational viewof a direct-viewing storage tube incorporating a storage targetstructure according to the present invention;

FIGURE 2 is a cross-sectional view in perspective of one embodiment of aportion of a storage-viewing target structure according to theinvention;

FIGURE 3 is a cross-sectional view of another embodiment of a portion ofa storage-viewing target structure according to the invention; and

FIGURES 4, 5 and 6 are explanatory diagrams to illustrate thefunctioning of the storage-viewing target structure of the invention.

Before proceeding with a detailed description of the invention, it maybe heipful at this point to explain the operation of a bistabledirect-viewing storage tube. With reference to FIGURE 4, a typicalbistable-storage-characteristic curve is shown where display brightnessis plotted against the storage-surface potential. Some of the electronsfrom the flood gun provide the display and others are responsible forregenerative action by which is meant restoring or maintaining thestored potentials). lt will be understood that there are two equilibriumdielectric potentials: (a) the potential of the flood gun cathode, orblack, and (b) the potential of the collector mesh, or white At thoseportions of the storage target surface which are at the potential of theood gun cathode, no iiood electrons can penetrate the storage target fordisplay purposes. At those portions or the storage target surface whichare at the potential of the collector-mesh, iiood electrons canpenetrate the storage target for display purposes. In the absence of awriting electron beam, the black and the white areas of the storagedielectric are maintained at their respective equilibrium potentials bythe low energy flood electrons. Thus, in the White areas, where thedielectric surface is charged to the co1- lector or positive potential,the iiood electrons strike at a velocity sufficient to produce asecondary emission ratio greater than unity; hence, more than onesecondary electron is released for every incident flood electron and, as

long as the collector electrode remains more positive than thedielectric surface, it will attract and collect the secondary electronsso that a net positive charging current will exist. Any tendency of thesurface to fall below the collector potential is thus compensated for bythe positive charging action of secondary emission. In the black areas,near zero-volt potential, the flood electrons strike the dielectricsurface with almost zero velocity and produce virtually no secondaryelectrons. I-Ience, any tendency of the surface potential to rise abovezero volts is compensated for by the negative charging action of tl eextremely low energy flood electrons. Surface areas with a potentialhaving any value between zero and that of the collector potential willbe driven in either the negative or the positive direction until theyreach one or the other of the two stable points.

As will be seen in FIGURE 4, a point, V0, represents the criticalpotential which divides the two regions of negative and positiveregeneration. T hus, in order to write a black surface to White, thewriting beam must raise the storage-surface potential to a value justbeyond this critical potential so that positive regeneration will carrythe surface the rest of the way to the collector potential. This isaccomplished by the writing beam which deposits a positive chargepattern by using the high secondary electron emission ratio of thedielectric at the writing beam energy. A stored pattern can be erased bymomentarily decreasing the collector potential in order to eliminate thebistable regeneration. If the collector potential is decreased to avalue slightly below the critical potential, Vo, positive regenerationis eliminated and the potential of the written area on the dielectricwill be reduced to zero volts (i.e., to black).

Referring now to FIGURES l and 2, a bistable directviewing storage tubeis shown which does not require a storage dielectric target separatefrom the viewing screen. The storage tube comprises an evacuatedenvelope 2 having a relatively large diameter bulbous portion 4 and anarrow neck portion 6. Disposed within the neck portion 6 is an electrongun 8 for forming a writing electron beam of elemental area. Thiselectron gun may be of conventional design and construction and includesa cathode 10, a beam-forming electrode section 12, and a deection system14 for causing the beam formed by the electron gun 8 to orthogonallyscan the target structure 16 disposed in the large bulbous portion 4 ofthe tube 2. Also disposed in the neck portion is a ood electron gun 18of conventional design and construction which is adapted to flood orblanket the target structure 16 with a relatively broad beam of lowenergy electrons.

The inner surface of the tube 2 may be provided with one or morecollimating lenses 20, 20 and 20 in order to collimate the floodelectrons from the ood gun 18 so that these electrons arrive at thetarget structure 16 at approximately normal incidence. The collimatingelectrodes may be constituted by conductive coatings of graphite, forexample, on the inside of the bulb 4.

In the embodiment of a storage tube according to the invention shown inFIGURE 1, a collector mesh electrode member 22 is disposed adjacent tobut spaced from the target structure 15. The collector electrode Z2 maycomprise an electroformed nickel screen 24 rather tautly stretchedacross the edge of a ring support member 26 and welded thereto. Thecollector mesh may typically have a thickness of about 0.0004 to 0.0008"and include about 250 openings per inch so as to be substantiallytransparent to electrons from the writing and flood beam electron guns 8and i8.

So far in the description of the present invention the elements andcomponents of a conventional direct-viewin g storage tube have beendescribed. However, as noted hereinbefore such conventional tubes havenext included two separate and spaced target structures-a storage targetmember and a viewing target or screen. In the present invention, thefunctions of these separate structures are performed by a single targetstructure with the mesh support structure of the storage target beingeliminated. This is achieved by providing the viewing screen structurein a conventional manner on the inner surface of the opticallytransparent end portion or faceplate 28 of the tube 2. Thus, the viewingportion of the target comprises an optically transparent conductivecoating 30 which serves as an accelerating electrode for the iloodelectrons which penetrate the storage target portion of the targetstructure i6. The accelerating electrode member 30 may comprise a layerof stannous oxide, for example, over which is disposed a phosphor layer32 of material capable of being excited into luminescence by theimpingement of electrons thereupon. Disposed on the phosphor layer 32 isa storage dielectric layer 34 which may be of any suitable electricallyinsulating material exhibiting secondary electron emission and which isporous or of very low density. Such a porous dielectric layer 34- may befabricated by evaporating the dielectric material onto the previouslysettled phosphor layer 32 under a very high gas pressure in the range of1000 to 2000 microns of mercury. Alternatively, the porous layer 34 maybe formed by loosely depositing dielectric particles by the well-knownsettling process. Almost any insulator material can be employed for thedielectric layer 34 providing it has a secondary electron emission ratiogreater than unity and suticient electrical resistivity i.e., at least1013 ohm-cm.). Typical examples of such dielectric materials are talcand magnesium fluoride. With a porous dielectric layer 34, many of theflood electrons arriving at the areas thereof which are the higher orpositive potential will enter the dielectric layer and proceed throughit and then be accelerated and strike the phosphor layer 32 causing itto luminesce at that point or area.

Because of the electrical resistivity of the porous dielectric layer 34,the surface thereof facing the ood gun i8 can be maintained at or nearthe potential of the flood gun cathode which is the black stable pointmentioned previously in describing the operation of storage tubes of thebistable mode of operation. Likewise, because of the resistivity of thedielectric layer 34, the side or surface thereof in contact with thephosphor layer 32 can be held at several hundred or even severalthousand volts higher in potential. Hence, areas of the dielectricsurface facing the tlood gun which are below the rst cross-overpotential of the surface (i.e., the potential at which the secondaryemission ratio is less than unity) are charged in the negative directionby the flood electrons and are maintained thereby at the black stablepotential near or at the flood gun cathode potential. Areas of thesurface of the dielectric layer 34 which are above the first crossoverpotential (that is, where the secondary electron emiss1on ratio isgreater than unity) are charged in the positive direction by the floodelectrons and are maintained thereby at the second or white stablepotential near or at the potential of the collector electrode 22. Thedielectric surface may be switched in point-to-point fashion from thelower or black stable potential to the higher or white stable potentialby the Writing beam produced by the writing electron gun 8 while theswitch from thehigher (white) potential to the lower (blacl may beaccomplished by means of the flood electrons in combination with alowering of the collector potential to a value below the firstcross-over potential.

It will thus be understood that in operation the tube of the presentinvention functions in a substantially similar manner as the bistabledirect-viewing storage tubes of the prior art. One of the main problemsencountered in operating a bistable tube according to the presentinvention, where the storage target portion serves as a viewing screenas well, is obtaining the desired light output since the collectorpotential in a bistable tube is usually around l5() volts with respectto the storage surface and a volt acceleration is not suicient toproduce useful luminescence. This problem is overcome by the presentinvention and the target structure shown by the use of the transparentconductive layer 36 which may have a potential of about one kv. thereon.

The theory of the operation of the combination storageviewing target ofthe present invention is as follows: with the collector electrodepotential at about 100 volts, the surface of the porous insulator layer34 is at the collector potential in the unwritten (dark) state. Thesecondary emission curve in FIGURE 4 shows this stable operatingpotential. The porosity of the dielectric layer 34 allows higher beampenetration (x) since the amount thereof is inversely proportional todensity according to:

where a=the density, V=the primary beam potential and k--a constant. Incontrast with the bulk dielectric storage targets of the bistablestorage tubes of the prior art, the density of a porous dielectric layeraccording to the present invention can be made by one or more of themethods described previously as low as 2% of the normal bulk insulator.The porous insulator layer therefore is charged by volume conduction tothe phosphor layer potential by the writing beam (which may be operatedat from 2 to 3 kv.) which can penetrate the porous insulator layer 34.Hence, once the writing beam has charged the dielectric surface to thepotential of the phosphor layer, flood electrons landing on thedielectric surface will have a beam energy of about l kv. Thus the floodelectrons can also penetrate the dielectric layer 34 and excite thephosphor layer 32 as well as maintain the higher potential by volumeconduction despite the fact that the collector electrode 22 is at thelower potential. This result is obtainable by making the thickness ofthe porous dielectric layer 34 less than the collector cell dimension.The porous dielectric layer must also have a higher volume conductionratio than the secondary emission ratio of the dielectric layer. Sinceporous dielectric layers having a volume conduction gain of greater thanare attainable with a secondary emission gain of 2 or 3, theseconditions are met.

With reference to FIGURE 5, the two stable operating potentials in termsof the secondary emission curve and the volume conduction curve can beseen. The volume conduction curves shown are for different electricfields, E. As the surface of the dielectric layer charges toward thephosphor potential, the electric gradient across the porous dielectriclayer varies so that the dotted curve is the actual gain curve thatapplies. As can be seen from this figure, the combined volume conductiongainsecondary emission gain curve crosses the unity gain axis to givethe second stable operating potential.

While in the embodiment of FIGURE l the collector electrode 22 has beenshown and described as an electron transparent electrically conductiveelectrode or mesh disposed adjacent to but spaced from the targetassembly 16, this is not necessarily the only arrangement possibleaccording to the invention. In fact, if collimation of the iiood beam isnot critical, as well might be the case in some applications, then thecollector might well be constituted by a ring (such as the collectorsupport ring 26 in FIGURE 1) surrounding the storage surface area. Inanother embodiment, shown in FIGURE 2, the collector electrode maycomprise a layer 36 of electrically conductive material such as aluminumdeposited directly on the surface of the dielectric layer 34 in the formof a metallic membrane having many separate voids constituting a largepercent (at least 50%) of the surface area of the dielectric layer 34.Thus the porous dielectric layer 34 will be exposed to the writing beamand the flood beam through these voids in the conductive collector layer36. Alternatively, it is possible to utilize a collector member 38 inthe form of a metallic mesh deposited on the dielectric layer 34 asshown in FIGURE 3. Such a Contact collector may be formed by evaporatinga metal through a mask onto a thin plastic lm and then transferrng theconductive screen thus formed to the surface of the dielectric layer 34.This transfer may be accomplished by immersing the plastic film in a vatfilled with water and causing the film to settle upon the top of thedielectric layer by decanting the Water from the vat. Thereafter theassembly is heated in order to evaporate or decompose the plastic filmleaving the thin conductive screen 38 in place on the dielectric layer34.

There thus has been described a bistable direct-viewing storage tubewhich uses a simplified and meshless storage target structure incombination with a viewing target and hence is easier and moreeconomical to manufacture.

What is claimed is:

1. A bistable directviewing storage tube comprising:

(A) an evacuated envelope having a viewing faceplate;

(B) a first electron gun disposed in said envelope for forming anelectron beam of elemental crosssectional area;

(C) a target structure in said envelope comprising:

(l) an electrically conductive light transparent tilm disposed on theinner surface of said faceplate;

(2) a layer of phosphor material disposed on said transparent film;

(3) and a layer of porous dielectric material selected from the groupconsisting of talc and magnesium fluoride disposed on said phosphorlayer;

(D) a second electron gun disposed in said envelope for iiooding saidporous dielectric layer with a broad beam of electrons;

(E) and an electron transparent electrically conductive electrode memberin the form of a metallic membrane having separate voids thereinconstituting at least fifty percent of the surface area of said layer ofporous dielectric material disposed on the surface of said porousdielectric layer for collecting secondary electrons emitted therefrom.

References Cited UNITED STATES PATENTS 2,777,060 l/l957 Waters 313-68 X3,284,652 ll/l966 Yaggy 313-68 3,293,473 12/1966 Anderson 315-12 X3,293,474 12/1966 Gibson 315-12 X FOREIGN PATENTS 949,330 2/ 1964 GreatBritain.

RQBERT SEGAL, Primary Examiner,

